The invention relates to improvements in the measuring elements of a closed circuit system for controlling and correcting the print in an offset printing machine, using densitometric measurements of each of the colors employed in the offset technique, with a single measurement for each color registration, whereby the component of the basic colors of each of the colors taking part in the printing is evaluated by decomposing each of the colors into red, green and blue. The evaluation is carried out simultaneously to obtain three values which are continuously compared with other values established in a memory program provided for each one of the valves of each one of the printing bodies.
Polychromatic printing by the offset system is determined by the successive addition of crosslinking points which reconstitute an image decomposed into parts comprising blue, yellow, red and black; the perfect positioning of these colors and the proximity of the crosslinking points permit the human eye to perceive a clear image containing the colors and tones of the original subject.
The proportion of each color is established from the start, when the printing molds are prepared, but the amount of color deposited on the paper during printing depends on the amount of ink and there is no rule or mathematic formula which permits the amount of ink that the printed image should bear in each one of the colors to be established.
The chromatic equilibrium in each case depends on the evaluative criterion of an expert who, after carrying out sufficient tests, determines the appropriate rule for the specific case in question.
Once the appropriate rule for that type of printing is established, a continuity should be maintained, wherefore the ink adjusting elements should be revised periodically. This verification in the quality and equilibrium of the ink also takes place subjectively, depending on the criterion of the expert and is carried out by sampling.
In practice, the temperature of the ink, when commencing printing, is determined by the room temperature of the premises. After a working time of approximately 1 hour, the friction and lamination undergone by the ink causes the temperature to be increased in about 5.degree. C., whereby the viscosity is dangerously reduced for the equilibrium of the colors. Thus, for example, an ink which at 22.degree. C. has a viscosity of 550 poises, will at 27.degree. C. have a viscosity of 375 poises, wherefore there is produced an increase in the output of the ink at the same velocity at which the temperature is increased, but not in proportion thereto.
These variations, presently uncontrollable, are practically impossible to detect, since the difference only appears if printed sheet 1 is compared with printed sheet 5,000, for example. Hence a continuous sampling takes place while printing.
Nevertheless, the printing speed of from 4 to 6,000 sheets/hour with plane offset and from 20 to 25,000 sheets with rotary offset, forces a constant action to be maintained on the installation.
On the other hand, to fairly evaluate the problem existing with respect to this question, it is necessary to bear in mind that the ink, in the system of partial discharge from the printers block, is controlled by an alignment of valves constituted by cutters of about 4 cms in length which attack the vibrator roller, leaving a narrow slit whose separation determines the amount of ink deposited during printing.
In a printing machine having a mold bearing cylinder which has a perimeter of 100 cms and a length of 1401/2 cms, there are arranged 36 cutters of the valves which permit a greater or lesser amount of flow of ink in a particular zone. For example, if there is a strip of the paper which should not be printed, it is necessary to close the printers block in the zone corresponding to that strip.
Thus, it is clear that it is not only necessary to control and modify the amount of ink in order to maintain a chromatic equilibrium, but it is also necessary to know to know at which specific zone of the printers block the correction should be made. For such an effect, the valves are identified in correspondence with identification zones provided on the printed papers, so that if an excess or a lack of color is observed at a particular spot on the printed sheet, the valve of the printers block which should be opened or closed to correct this error can become known.
Bearing in mind that the addition of further ink modifies the characteristics of that which remains in the printers block, it can clearly be seen that the printing machine should be serviced with respect to the control of the print.
Hence, it is not surprising that this industry, urged by the interest of the printers, attempts to find and put into practice means capable of automatically fixing, firstly, the parameters between which the quality of the print should be maintained, and subsequently maintaining such parameters throughout the run, without any human intervention.
The publication DER POLYGRAPH has occasionally dealt with this subject giving rather theoretical general information of possible solutions adopted concerning this question. In an article referring to the INKATRON system, it indicates the provision of a fixed scanner head which is provided with an obturator synchronized with the operating speed of the machine by means of an electric chronometer mechanically joined to the printing cylinder. The scanner head takes three values: one of the white value of the paper; a second of the density of the color of the ink, and a third of the black value. Subsequently, the white value with respect to the density of the color is compared in a computer with the white value with respect to the black value, these values resulting in a voltage which is proportional to the density of the color independently, in theory, of the illuminating power or the sensitivity of the photoelectric cell.
However, this principle has not been feasible in practice and no installation is known in which it has been applied, not even partially, since the use of a fixed scanner head and a mechanical obturator synchronized with the operating speed of the machine only permit a narrow strip of paper to be measured. This can never give a clear idea of the complete print which, as is known, depends on the control of 36 zones of the printers block.
Another article, also publisehd in DER POLYGRAPH, this time referring to the GRETAG-T 61 installation, discloses a measuring system based on a densitometer, whereby each of the colors of the scale and three tones of black is measured, i.e. three measurements per color, and density values are obtained by means of the light reflected by the strip of paper which is converged by an optical scanner which, provided with a filter, feeds a semiconductor photoreceiver which generates an electric current proportional to the amount of light. The needs of the printer are not satisfied by this measuring system, since a total of 12 successive measurements should be taken, inserting a filter in each case, which requires a complicated mechanics, wherefore the assembly is prone to failures and, therefore, hardly reliable.
To eliminate the unreliability of the preceding system, other means have been searched for, such as that disclosed in German patent No. 2,238,007 wherein a scanner head formed of three phototransistors, in front of each one of which there is arranged an optical filter having a narrow strip, whose by-pass zones differ from each other. Thus, the print is directly scanned, wherefore different measurements should be taken, preferably outside the machine, adopting again the sampling system and avoiding the principle of continuous control of the print with the slowness inherent thereto and the inclusion of the error contained therein.
This avoidance is clear when the invention is compared with the object of German patent No. 2,203,145 according to which one or more, but not all, of the columns of the printing product are scanned, in other words, some of the 35 columns into which, in practice, the inking roller is divided by the control valves of the printers blocks.
Differing from other patents to which reference is now made, measuring takes place by reflection and transparency, complicating it even more, if possible, to finally obtain confused values since the transparency, for example, is very variable on the same printed sheet mainly due to the paper itself.
These German patents avoid the commonly used practical means for measuring, the densitometer, and instead employ other means. However, although German patent No. 2,238,007 considers the densitometer as an element pertaining to the prior art and can, therefore, be replaced by other more modern techniques, the use of filters are maintained and three measurements are taken, one for each color. Thus, the prior art is hardly improved and the invention does not represent a major advance which, on the other hand, is dubious.
U.S. Pat. No. 2,968,988 describes an apparatus for detecing the density of the ink which comprises a photoelectric scanner head which is transversally movable to the print, wherein the reading is taken on colored stains.
The stains are scanned by the photoelectric head after the sheet of paper has passed along the printing cylinder, thereby detecting the value of the reflection density of a given tone and showing them to the operator so that he may, depending on his criterion, apply the values of the ink and correct the tone of the print.
The values which the operator detects are marked by a recording galvanometer which prints, on a strip of paper, zones for the different colors, each zone including the repeating configuration which represents the variation in the density of the ink for the color in question on the width of the sheet.
Subsequently, the same inventor improved the apparatus in U.S. Pat. No. 2,969,016, including filters using a single photoelectric cell which is enclosed in a rotary obturator, one part of its circumference being provided with an opening situated in such a way that the light reflected from the colored stains can pass successively through the opening and can strike against the photoelectric cell. The obturator rotates synchronously with the printing cylinders which, logically, complicates the system.
On the other hand, in accordance with the invention, the photoelectric cell is furthermore exposed to two illumination intensities having a different standardization and the rotary obturator encloses the photoelectric cell during a part of each rotation, emitting a prior control signal followed by a measuring signal which is registered by indication instruments as in the prior invention.
Both cases do not detect what part of the printers block sends a greater or a lesser amount of ink than necessary and, therefore, utilization of the invention is doubtful, so much so that it has not, in practice, been applied to any offset printing machine.
Another attempt to solve the problem appears in U.S. Pat. No. 3,736,725, wherein, returning to the densitometer as the measuring element, complementary filters are used by means of which the colors are measured independently, using an illumination from a discharge lamp similar to a stroboscope which is synchronized with the advance.
However, the measurements which are obtained do not indicate in which part of the printers block an excess or a lack of ink is being produced in each one of the colors.
Other patents, such as U.S. Pat. No. 3,970,394, describe this problem trying to solve same with even more sophisticated systems. In the case of this patent, the problem is solved by using optical fibers and reflection filters, the optical fibers acting as detectors.
The importance of this subject is clear from the aforegoing data since, different persons in different countries have dedicated their time in finding a solution thereto. There is no doubt that a thorough search of prior art would bring to light a high number of patents wherein other persons in other places have also sought solutions. However, since the correct solution to this problem is a general concern of printers and manufacturers of machines for graphic arts, it is known that the solution, if it exists, would have been published by specialized informants (DER POLYGRAPH, for example) and the installations would have been exhibited in International Fairs. Thus, one can summarize the prior art as being the following:
Use of a densitometer as the measuring element. PA0 Use of photosensitive cells as the measuring element. PA0 Use of complementary narrow band filters, etc., for selecting the measurement. PA0 Use of synchronous means to effect measuring in a machine during the printing process. PA0 Use of different measurements for each color. PA0 Measuring zones of the print. PA0 Measuring the complete print. PA0 Measuring colored stains. PA0 Measuring with a fixed measuring head. PA0 Measuring with a movable measuring head. PA0 Measuring data plotted graphically on a strip of paper. PA0 The constant attention of a specialist is required. PA0 The specific valve or valves which should be controlled in each case is not indicated. PA0 The correction should be made manually and then the result should be awaited.
Since the real needs are to know the degree of distortion before it is produced in a visually appreciable manner, to automatically effect the correction, storage of data for subsequent issues, etc., it is clear that the prior art does not solve the problem due to the following reasons: